1. Field of the Invention
The present invention relates generally to an electric drive unit for a solenoid-operated control valve of a variable displacement compressor. More particularly, it relates to an electric control circuit unit for controlling the energization of a solenoid element of a solenoid-operated capacity control unit of a variable capacity refrigerant compressor for use in a car air-conditioning system.
2. Description of the Related Art
A variable displacement wobble plate type compressor is known as a typical variable capacity compressor adapted for incorporation in a car air-conditioning circuit to compress a refrigerant gas. U.S. Pat. No. 4,747,754 to Fujii et al discloses a variable capacity wobble plate type compressor with a solenoid-operated wobble angle control unit, in which a solenoid valve is used for closing and opening a fluid passageway between a crankcase chamber provided for accommodating a drive and a wobble plate assembly operatively connected to a piston mechanism compressing the refrigerant gas and a discharge chamber for receiving therein a compressed refrigerant gas. The communication between the crankcase and the discharge chamber through the fluid passageway opened by the solenoid valve increases a pressure level in the crankcase chamber, and an interruption between both chambers due to closing of the fluid passageway by the solenoid valve decreases a pressure level in the crankcase. The increase and decrease of the pressure level in the crankcase chamber causes a change in an angularity of the drive and wobble plate assembly with respect to a small angularity erect position, thus causing a change in a reciprocating stroke of the piston mechanism which, in turn, causes a change in a compressing capacity of the compressor. Therefore, the combination of the solenoid valve, the fluid passageway, and the drive and wobble plate assembly is considered to be a solenoid-operated capacity control unit of the variable capacity type wobble plate type compressor.
U.S. Pat. No. 4,586,874 to Hiraga et al, discloses another type of capacity control system for a variable displacement wobble plate type compressor, in which the solenoid-operated valve is arranged so as to be capable of opening and closing a fluid passageway between a crankcase chamber and a suction chamber of the compressor. That is, when the fluid passageway is opened by the solenoid valve unit, a decrease in pressure within the crankcase chamber occurs so that the angularity of the wobble plate can be increased. An increase in pressure in the crankcase chamber is caused by a high pressure blow-by gas leaking from the cylinder bores, to permit the compressor pistons to reciprocate to compress the refrigerant gas.
In the above-mentioned typical conventional variable capacity compressors, when the solenoid valve is electrically energized by the supply of an electric start voltage, to thereby open or close the fluid passageway, the opened or closed condition of the fluid passageway is usually maintained for a certain period of time, i.e., the energization of the solenoid of the solenoid valve must be retained by the supply of the same electric voltage as the initial electric start voltage supplied thereto during that period. More specifically, the conventional design of the solenoid-operated capacity control valve is based on the principle that, after the switching of the solenoid-operated valve is completed, the high starting voltage required to start the switching motion of the valve is continuously maintained to energize the solenoid of the solenoid-operated valve until de-energization of the solenoid is required. Note, the electric starting voltage must be appreciably higher than that required for retention of the energization of the solenoid, and therefore, the heat generated by the solenoid is very high, and thus the temperature of the solenoid becomes very high. In the case of the variable capacity refrigerant compressor used in a car compartment air-conditioning system, the compressor is driven by the car engine via a rotation transmitting system, and a solenoid clutch device is mounted in the engine compartment where the temperature is very high during the operation of the engine, and therefore, the temperature of the solenoid per se may reach 200.degree. C. or higher due to the heat generated by the high energizing voltage and the temperature of the engine compartment. This high temperature causes problems such as a reduction in an electromagnetic force of the solenoid due to an increase in the electric resistance of the solenoid affected by the high temperature, and a reduction in the strength of the plastic materials used for the molding and insulating of the wiring of the solenoid. Further, the car battery must supply the high electric power for maintaining the energization of the solenoid of the solenoid-operated capacity control unit of the compressor, in addition to the electrical power supply required for actuating the above-mentioned solenoid clutch to couple the compressor to the car engine.